The present invention relates to a secondary ion mass analyzing apparatus and in particular to a secondary ion mass analyzing apparatus which is suitable for performing a depth directional analysis of a specimen at a high accuracy.
Prior art secondary ion mass analyzing apparatuses have been widely used for depth directional analysis of a specimen. Electronic aperture method has been widely used for enhancing the accuracy of the analysis in a depth direction. The electronic aperture method includes raster-scanning a primary ion beam on a specimen to etch the specimen uniformly, gating a detection system so that secondary ion from etching crater edges different in depth will not be introduced to a mass analyzer, introducing signals from only the central portion of the etched crater to the mass analyzer to integrate them, whereby the accuracy of the depth directional analysis, that is, the dynamic range in a depth direction is enhanced.
In accordance with this electronic aperture method, the specimen is subjected to two dimensional scanning of a primary ion beam along X and Y axes. This causes the specimen to be etched so that the etched portion becomes a crater. Secondary ions are generated from the crater and are detected.
Among the detected secondary ions, the secondary ions from the edge of the crater are eliminated by an electrical gate, that is, an electronic aperture so that substantially only the ions from the central portion of the crater, that is, only the necessary secondary ion beam is introduced to the mass-analyzer in which it is mass-analyzed. This enhances the analysis accuracy of the specimen in a depth direction and widens the dynamic range.
It is known that the ion density distribution of primary ion beam generally exhibits a Gaussian distribution characteristics. However, the density distribution of primary ions in fact exhibit a distribution having a skirt remarkably broader than that of the Gaussian distribution due to variations in initial energy of the primary ions and collision with residual gas molecules and scattering thereby. Therefore, introduction of the secondary ions generated from the periphery of the crater into the mass analyzer is inevitable even if the electronic sperture is opened only when the primary ion beam is positioned in the center of the crater.
FIG. 1 is a view showing the above mentioned prior art electronic aperture method.
In FIG. 1, a crater 3 is formed on a specimen 2 by etching if a primary ion beam 1 is raster-scanned on the specimen 2.
It is known that the ion density distribution 4 of primary ion beam 1 generally exhibits a Gaussian distribution characteristics. However, practical density distribution of primary ions exhibits a distribution having a skirt remarkably broader than that of the Gaussian distribution due to variations in the initial energy of the primary ions and collision with residual gas molecules and scattering thereby. Although a detection gate is opened in the electronic aperture method when the center of the primary ion beam 1 is located at a secondary ion capture area 5, the captured secondary ions inevitably include unwanted secondary ions from the periphery of the crater edge 6 due to existence of the skirt of the density distribution of the primary ion beam 1.
FIG. 2 is a view showing an imaging condition of a secondary ion emission pattern in a prior art method.
In order to overcome the above mentioned problems, there is an approach in which an emission pattern of the secondary ions 7 emitted by irradiation with the primary ions 1 is imaged on a position of an entrance slit 9 of a mass analyzer by means of an extraction electrode 10 and a transfer lens 8 so that the field of the emission pattern is restricted by the dimension of the entrance slit and only such ions as do not include any ions from the crater periphery parts are introduced to the mass analyzer by restricting the dimension of the beam and the secondary ions from the periphery of the beam distribution. However in this approach, it is an essential requirement that the emission pattern of the secondary ions 7 emitted from the specimens 2 be exactly imaged on the position of the entrance slit 9 by means of the transfer lens. An approach including forming an image of a specimen 2 of secondary ions 7 generated from the specimen 2 on a predetermined position by a lens, providing an aperture at this position and taking only secondary ions from a specimen area corresponding to the central portion of the primary ion beam 1 by this aperture for preventing the introduction of the secondary ions 7 generated from the periphery of the crater 3 into a mass analyzer is known from Japanese Patent Publication 53-14935. This approach will be referred to as unnecessary secondary ion removing technique using lens and aperture.
However, in the above mentioned prior art approach a secondary ion 7 emission pattern is imaged on the entrance slit 9. Means for setting and confirming detailed conditions for imaging an emission pattern on the entrance slit 9 is not considered. There is a problem that effects by a structure cannot be exhibited.
In other words, setting of conditions of the lens 8 can be determined based on only the strength of the secondary ions 7 passing through the entrance slit 9. The conditions of the lens 8 are preset in such a manner that a maximum amount of the secondary ions 7 can pass through the entrance slit 9.
However, means for confirming whether or not an image of the specimen is formed at the position of the aperture is not considered. It is determined that ions collide with nothing and the ion orbit is correct if the amount of ions is more than a reference value. There is a problem that inherent effects of unnecessary secondary ion elimination technique using lens and aperture may not be sufficiently obtained.